The present invention relates to a safety switch plunger and a safety switch.
Safety switches are well known, and are typically used to prevent access to for example electromechanical machinery when that machinery is in operation. In a conventional arrangement the safety switch is mounted on a doorpost of a machinery guard, and an actuator for the safety switch is mounted on a corresponding door. When the door is closed the actuator engages with the safety switch, which in turn closes a set of electrical contacts which allow power to be supplied to the machinery. This arrangement ensures that power can only be supplied to the machinery when the guard door is shut. When the guard door is opened, the actuator disengages from the safety switch, thereby opening the electrical contacts and cutting off the supply of power to the machinery.
A typical safety switch comprises a housing, in which is provided a set of contacts fixed in position relative to the housing. An axially slideable plunger is mounted inside the housing, and is moveable relative to the housing. The plunger is provided with another set of contacts. The plunger is biased towards a cam arrangement by a spring. The actuator mentioned above is arranged to engage with the cam arrangement.
In many safety switches, if the actuator is not engaged with the cam arrangement (i.e. if the actuator is not engaged with the safety switch), the cam arrangement is arranged to prevent the contacts on the plunger coming into contact with the contacts of the housing by preventing movement of the plunger (i.e. the plunger is kept in a first plunger position). By preventing the contacts from contacting one another, the switch cannot conduct electricity while the actuator is engaged with the cam arrangement.
Bringing the actuator into engagement with the cam arrangement causes the cam arrangement to rotate, which in turn causes the plunger (which is biased toward the cam arrangement) to move into a notch provided in the cam arrangement. The plunger is then in a second plunger position. When the plunger moves into the notch, the contacts on the plunger are brought into contact with the contacts of the housing, allowing electricity to flow through the safety switch.
Should the cam arrangement fail (e.g. become worn, cracked, or be displaced etc) the spring may cause the plunger to move further than intended (i.e. to a third plunger position). In the above example, if the cam arrangement is removed (for example due to an impact on the switch) the spring will still bias the plunger toward where the cam arrangement would have been. Thus, even when the cam arrangement is not present, the contacts of the housing may be brought into contact with the contacts of the plunger, allowing electricity to flow through the switch. In summary, if the cam fails, the switch defaults to the situation where it supplies electricity. This is known in the industry as a ‘fail to danger’.
If the cam arrangement fails when the door to the machinery guard is open, the switch will conduct electricity, and machinery within the guard will be either powered, or powered and operating. This is undesirable, since the purpose of the safety switch is to only allow electricity to be supplied to the machinery when the door to the machinery guard is closed.
If the cam arrangement fails when the door to the machinery guard is closed and the machinery is in operation, it may not be possible to identify the failure until the door has been opened. When the door has been opened, the machinery will continue to operate, highlighting a failure with the safety switch.
It is desired to overcome or substantially mitigate the above-mentioned disadvantages.